In a workpiece pilot hole, an internal thread can be produced by thread cutting or by non-cutting thread shaping. In the process of non-cutting thread shaping, the workpiece material is plastically deformed on the pilot hole wall, thereby forming an internal thread.
A conventional method initially comprises providing a workpiece pilot hole in which an internal thread is produced by means of a thread producing tool (that is, a thread cutting or thread shaping tool), which internal thread comprises an thread major diameter and a thread minor diameter. In a first process step, the thread producing tool is introduced into the workpiece pilot hole in a thread producing stroke with a defined thread producing feed and a synchronized thread producing speed until it reaches a target thread depth, by which action the internal thread is formed. A reversing stroke in the opposite direction follows, in which the thread producing tool is moved out of the internal thread of the workpiece pilot hole, said stroke having a feed in the opposite direction and a synchronized reversing speed, such that the thread producing tool can be removed from the pilot hole along the pitch of the internal thread in substantially stress-free operation.
The internal thread shaped in this manner in the workpiece pilot hole has a radially outer thread base and a radially inner crest, which are spaced apart by one profile height in the radial direction of the hole. As a result of the plastic deformation of the workpiece material that specifically occurs in a non-cutting thread shaping process, defects or material weakenings can form in the shaped internal thread. These occur particularly on the radially inner crest of the internal thread and impair the setting behavior of a screw member screwed into the internal thread.
DE 2 058 991 discloses a thread shaping tool for forming internal threads which comprises a cutter head for precision machining or rough reaming a workpiece pilot hole, respectively. DE 79 22 782 U1 discloses a combined tool for drilling and thread shaping, which allows drilling and shaping a thread in a single operation.
DE 696 21 092 T2 discloses a thread producing tool of this generic type which is used to initially produce an internal thread profile on the pilot hole wall. The internal thread profile comprises the final thread major diameter as well as an thread inner diameter which is smaller than the thread minor diameter in the finished state of the internal thread. A final processing step follows in which the thread inner diameter of the internal thread profile is widened to the final thread minor diameter of the internal thread. The internal thread profile is widened by machining, for example by grinding or drilling, in which process the internal thread profile is ground or drilled to the thread minor diameter.